The present invention relates to a semiconductor diode and particularly to one comprising a semiconductor body including at least two adjoining regions of the same conductivity type and of different resistivities.
Known photoelectric detectors comprise, in addition to photo-multipliers having a high degree of linearity and photodiodes with high current, photo-multipliers of which the part where multiplication occurs constitutes a diode or photo-counter which is usually manufactured from semiconductor material.
One of the required properties of a semiconductor diode for this application is a linear response within a wide range, in particular with a high current level. As is known, one of the important characteristic features in this respect is the value of the capacitance of the depletion zone formed at the photosensitive junction when polarized in the reverse direction. The capacitance of the depletion zone plays a part in determining the time constant (which must remain constant within a voltage range which is as large as possible) and remains substantially constant dependent upon the voltage difference between the depletion voltage and the breakdown voltage. In order to obtain a linear response in a wide range, the breakdown voltage must thus have a high value, whereas the value of the depletion voltage must be as low as possible.
The prior art includes diodes comprising a silicon substrate and obtained by inverted epitaxy which consists of depositing a thick, highly-doped epitaxial layer of a given conductivity type on a high purity monocrystalline substrate that has the same conductivity type, but a high resistivity (the epitaxial layer providing good mechanical rigidity), after which the thickness of the substrate is reduced. The diode is then formed by diffusing impurities of the opposite conductivity type into the remaining part of the substrate. Diodes obtained by this method exhibit drawbacks, mainly in that a very large concentration gradient is formed at the interface between the substrate and the epitaxial layer so that the breakdown voltage cannot assume a high value. In addition, the treatments of the substrate surface prior to depositing the epitaxial layer may give rise to interference of the crystal lattice of the epitaxial layer and the resulting irregularities of the crystal may involve a reduction of the breakdown voltage.
German Offenlegungsschrift 1,806,624, to which corresponds U.S. Pat. No. 3,534,231, issued Oct. 13, 1970, to J. R. Biard, discloses a semiconductor photo-detector which constitutes three semiconductor layers located one on the other, the first two layers constituting the photosensitive junction and the highly doped third layer being present at a distance from the photosensitive junction which is smaller than a diffusion length of the minority charge carriers. The photo-detector structure disclosed there is said to reduce leakage currents resulting from the occurrence of minority charge carriers, by providing below the photosensitive junction, a barrier layer, for example, in the form of a reverse biased second junction. The value of the breakdown voltage is of no significance in the structure there and the three layers of such structure can thus be obtained by conventional means without special precautions, with the exception that the distance between the first and the third layers must be smaller than the diffusion length of the minority charge carriers.
In contrast with the Biard disclosure, the present invention provides a diode exhibiting a considerable increase in breakdown voltage. The present invention, furthermore, reduces the drawbacks mentioned above with respect to inverted epitaxy.